1. Field of the Invention
The present invention generally relates to electrical connectors and, more particularly, to a flexible interconnect cable having improved design flexibility and three-dimensional conformity.
2. Description of the Related Art
Thin, flexible cables for interconnecting electrical devices are well-known in the art. These articles, variously referred to as flex cables, flex circuits, flex circuit cards, or flex cable assemblies, are particularly useful for carrying electrical signals in compact electronics applications. In the computer industry, cable assemblies are used for many purposes including power distribution, low-speed signal communication, and high-speed signal communication. For example, in high-performance computer systems, specialized cable assemblies are used to transmit high-speed signals from one processor to another processor, from a processor to system memory (RAM), or from a processor to an input/output (I/O) device. These specialized high-speed cable assemblies are mechanically and electrically connected to the printed circuit boards that support these components.
A typical flex cable has many conductive traces (thin conductive layers) placed on an insulative substrate, in a pattern appropriate for the particular interconnection, to form an elongated and flexible circuit structure. The conductive traces can be formed of any conductive material, as gold or copper, and the substrate can be any flexible material, usually a durable polymer, including polyester or polyimide, such as MYLAR or KAPTON, flexible insulation films. The conductors can be coated with an overlying layer of insulative material or hermetically sealed. Electrical contacts (or pads formed on the conductive traces) are provided at the ends of and along the conductive traces, including signal and ground plane contacts.
Interconnection with the contacts or pads formed on the flex cable may be provided in a temporary manner, e.g., using a switch or other mechanism that results in a physical wiping action by a component contact to achieve connection with the flex cable contact, or in a permanent manner, e.g., soldering. The contacts may be provided for through hole connection or surface-mount connection. A flex cable can have a multi-layer construction, i.e., conductive traces and contacts on upper and lower surfaces of the substrate or buried in the substrate. More complicated three-dimensional conductive traces can be constructed by, e.g., laser ablation or etching operations. Multiple flex cables can be used in parallel, stacked, and staggered assemblies.
FIG. 1 depicts a conventional flex cable design 10 for interconnecting a computer system to a CD-ROM (compact disk, read-only memory) device. Flex cable 10 includes a plurality of conductive traces 12 formed on an insulative substrate 14. The conductive traces terminate in through-holes 16 which have been plated with a conductive material. Two sets of holes define first and second connectors 18 and 19 at one end of the cable, and a third set of holes defines a third connector 20 at the other end of the cable. A fourth set of holes defines a fourth connector 22 at the end an extension 24 integrally formed with substrate 14. Conductive traces may be provided on the underside of the substrate (not shown) for connection to, e.g., the fourth connector 22.
The propagation delay for high-speed signals between electronic components affects overall performance of the computer system, so the propagation delay is decreased by reducing the total path length for the signal to travel, as well as by improving the dielectric properties of the cable assemblies. Reducing the path length is also generally desirable to provide more compact systems. Using shorter flex cables can, however, introduce other problems. Short flex cables have some ability to conform along the length of the flex but little flexibility perpendicular to the cable surface; for example, the flex cable 10 of FIG. 1 can easily flex (or bow) between the ends at connectors 18 and 20, but it has little ability to conform in the left-right direction. The ability to accept rotation from connector to connector is thus limited by the bending characteristics of the flex cable. Cable stiffness places stresses on the flex cable assembly and mating hardware when flex is bent or any dimensional mismatch occurs, and so can lead to defects if sufficient tolerance is not provided, and limits packaging density.
Because of the foregoing limitations, different cable designs are made to fit each application. For example, even with a single product, a first cable may be needed for system test, and a second cable needed for manufacturing volumes. Cable and hardware stresses are reduced by altering the construction of the cables, but this impacts electrical performance, presenting additional tolerances must be considered. Cable routings must also be based on standard flex characteristics. It, therefore, would be desirable to design a flex cable having improved physical conformity to lessen tolerance demands. It further would be advantageous if the improved flex cable provided increased interconnection design flexibility for additional functionality.